Low power CD-ROM player for portable computer

ABSTRACT

A computer subsystem of a computer includes a CPU, RAM, display, storage device, input device(s), and a digital-audio generating IC. A CD-ROM subsystem of the computer includes a CD-ROM drive and CD-ROM control buttons for controlling CD-ROM drive&#39;s operation while playing audio CDs. An audio-interface IC of the CD-ROM subsystem couples a bus of the computer subsystem to the CD-ROM drive, and to the control buttons. The audio-interface IC, in one operating mode, relays commands and data between the bus and the CD-ROM drive. A second operating mode permits turning the computer subsystem off while the audio-interface IC autonomously responds to the control buttons and transmits commands to the CD-ROM drive for playing an audio CD.

CLAIM OF PROVISIONAL APPLICATION RIGHTS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/079,508 filed on Mar. 26, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to digital computers and, moreparticularly, to a digital computer adapted for low power operationwhile playing a CD-ROM

2. Description of the Prior Art

Portable computers (i.e., notebook, laptop, palmtop and the like) frommajor original equipment manufacturers such as Toshiba, Compaq, Dell,IBM and others offer CD-ROM drives as either standard or optionaldevices. Notebook, laptop, palmtop computers are aimed at the mobilecomputer user who needs or wants to take work home from the office or ona business trip. An added benefit of CD-ROM equipped portable computersis the opportunity to enjoy periods of relaxation and pleasure byplaying audio tracks from standard music CDs. In the ensuing discussion,the term notebook computer will be understood to apply also to laptop,palmtop and other portable, battery powered computers.

The Windows operating system's media player or third party audioapplication can play back standard audio CDs on a portable computer.However the simple function of playing an integral audio CD-ROM requiresthat the entire notebook system be powered for the duration of the audioplay back. This causes excessive drain on the notebook's battery powersystem, unnecessarily consuming battery energy better saved for CPUintensive use such as word processing and spreadsheet analysis.

Conventional laptop and notebook computers typically have several powerdown modes. They can be powered down such that the CPU is almostcompletely off, with the state of the CPU saved on a hard drive. A verylow power portion of the CPU or an auxiliary circuit (e.g. keyboardcontroller) is typically used to recognize when a key is pressed. Thesystem then reactivates normal power to allow the CPU to retrieve thestored machine state from the hard drive thereby restoring the computerinto an operating mode. Some well known power saving modes are calledsleep mode, suspend mode and the like.

Consequently, a modern energy efficient computer will, over time,operate in several different power management regimes. For example, if aportable computer is being used in an office environment whereelectrical power consumption is an insignificant concern, then thecomputer user may want the computer to provide the highest performanceand availability possible. Conversely, if the computer is being operatedon battery power where there is no convenient source of electricalenergy, then the computer user may want to choose a power managementregime for the computer that will maximize the time the computeroperates without recharging its batteries, even though performance andavailability may be noticeably reduced.

To facilitate controlling electrical power consumption in personalcomputers, Intel Corporation, Microsoft Corporation, and ToshibaCorporation have jointly established an Advanced Configuration and PowerInterface Specification (“ACPI Specification”). The ACPI SpecificationRevision 1.0 of Dec. 22, 1996, Copyright 1996 Intel Corporation,Microsoft Corporation, Toshiba Corporation, establishes both a set offive (5) Global System States G3—Mechanical Off, G2/S5—Soft Off,G1—Sleeping, G0—Working, S4—Non-Volatile Sleep, and a set of four (4)Device Power States D0—Fully On, through D3—Off. The ACPI Specificationdefines the Global System States as follows.

G3 Electrical power is mechanically turned off.

G2/S5 Electrical power is turned on but the computer consumes a minimalamount of power by not executing either user or system computerprograms, and the system's context is not preserved by hardware.

G1 Electrical power is turned on, the system's context is preserved byhardware or system software, but user computer programs are not beingexecuted.

G0 Electrical power is turned on and user computer programs areexecuted. In the G0 state, devices such as hard disk drives, CD-ROMdrives, floppy diskette drives, etc. are dynamically turned on and offas needed.

S4 Electrical power may either be turned off, i.e. Global State G3, orturned on with the computer consuming a minimal amount of power, i.e.Global State G2/S5, while system context is preserved in a non-volatilestorage file before entering either the G3 or G2/S5 state, therebypermitting the computer to be restored to its prior operating state,i.e. G1 or G0.

The ACPI Specification further defines Device Power States as follows.

D0 The device is completely active and responsive, and consumes the mostelectrical power.

D1 A lower power state that is defined for different types of deviceswhich preserves more device context than the yet lower power state D2.

D2 An even lower power state than D1 that is again defined for differenttypes of devices, and which preserves less device context than state D1.

D3 Electrical power is fully removed from the device, device context islost, and system software must reinitialize the device when it is turnedon again.

The different computer operating modes and associated power managementregimes described above are each characterized by a unique power demand(i.e., current drain) from the battery power supply. This is animportant feature both in design of portable computer systems, and inmarketing them as well. Great attention is focused on minimizing thepower demand for each of the different Global and Device operatingmodes. Thus, the power demand characterizing each power managementregime is a critical factor to be considered for portable computers,particularly one that includes a CD-ROM drive for playing audio CDs.

In implementing conventional computer power management strategies, apower management routine (“PMR”) executed by the CPU must periodicallymonitor peripheral devices to assess whether a peripheral device'soperation may be suspended. Similarly, if it becomes necessary to accessa peripheral device whose operation has been suspended such as in DevicePower modes D1-D3, the PMR must resume that peripheral device'soperation. Generally, suspending the operation of a peripheral deviceand resuming its operation respectively require that the PMR executed bythe CPU perform a unique sequence of operations in turning offelectrical power to a peripheral device, and in turning electrical powerback on. Writing a computer program that detects a need to execute apower-on or a power-off sequence of operations for a peripheral deviceis a cumbersome task.

Previous portable computers that include a CD-ROM use PMR functions tominimize battery drain. However, if CPU operation has been suspended tosave electrical power, the computer can essentially do nothing.Therefore, in the minimal power drain mode, the CPU cannot use theWindows operating system's media player or third party audio applicationto play audio CDs.

A significant power drain in portable computers occurs in theconventional LCD monitor. Typically, 60 to 70% of the power consumed bya notebook is consumed by the display. Thus even if a computer'sdevices, including even perhaps the CPU, were in a lower power state,i.e., one of the lower Device Power States D1-D3 for power savingsduring CD-ROM play only, the need to use the normal LCD to displayCD-ROM status and the music playing status would itself impedesignificantly reducing power consumption.

For the reasons described above, it is apparent that a disadvantage ofpresent portable computers for playing audio CDs is that some portion ofthe computer system must remain energized state to detect key actuationand then to restore power or activate a power restore function of theCPU and associated peripherals (e.g. hard drive, keyboard controller,display, etc.). At times when a portable computer is being used duringtravel, or when line power is otherwise unavailable, the user may wishto play some audio CDs. Given the limited battery life of mostportables, e.g., 3 to 5 hours of use, the user may have to choose toforego using the CD-ROM capability for very long, out of fear that thenotebook will not be functional for needed work or communication.

BRIEF SUMMARY OF THE INVENTION

The present invention extends the playing time for a CD-ROM equippednotebook computer while minimizing the loss of potential operating timeas a computer.

Another advantage is to select and control the music being playedwithout engaging (powering on) the CPU or other notebook peripherals,i.e., hard drive, display, memory and the like.

Another advantage is to extend playing time and lower power drain whileminimizing the requirements for additional software drivers.

Another advantage would be to provide CD-ROM/music status to the userwithout using the normal display screen with its typically high batterydrain requirements.

Another advantage of the present invention is that it facilitatesportable computer product differentiation by providing various differentaudio CD playing user interfaces.

Another advantage of the present invention is that it permits designersof portable computer systems to choose among various different audio CDplaying user interfaces.

Another advantage of the present invention is that all of the precedingadvantages may be obtained merely by inserting an IC in accordance withthe present invention into an existing portable computer design.

In one embodiment the present invention is a digital computer thatincludes both a computer subsystem and a CD-ROM subsystem. The computersubsystem is conventional and includes a digital computer bus via whichvarious digital computer devices included in the computer subsystemexchange commands and data. Devices included in the computer subsysteminclude a central processing unit (“CPU”), a random access memory(“RAM”), a display, a read-write mass storage device, a manual inputdevice, and a digital-audio generating integrated circuit (“IC”).

The CD-ROM subsystem includes a conventional CD-ROM drive and an audiooutput amplifier that is coupled to the CD-ROM drive for receiving ananalog audio signal from the CD-ROM drive. The CD-ROM subsystem alsoincludes several CD-ROM control buttons for controlling operation of theCD-ROM drive during replay of audio compact disks (“CDs”). Anaudio-interface IC, also included in the CD-ROM subsystem, is coupled tothe digital computer bus of the computer subsystem, to the CD-ROM drive,and to the CD-ROM control buttons. The audio-interface IC, in a firstoperating mode in which the computer subsystem is energized andoperating, relays commands and data between the digital computer bus ofthe computer subsystem and the CD-ROM drive. In a second operating modein which the computer subsystem is not energized and is inoperative, theaudio-interface IC autonomously responds to signals received from theCD-ROM control buttons and transmits commands to the CD-ROM drive whichcause the CD-ROM drive to play an audio CD present in the CD-ROM drive.

In a particularly preferred embodiment the present invention theaudio-interface IC has a third operating mode in addition to the twodescribed above. In this third operating mode the computer subsystem isenergized and operating, and the audio-interface IC receives commandsfrom the CD-ROM control buttons and stores such commands for subsequentretrieval by a computer program executed by the CPU included in saidcomputer subsystem. Furthermore, in this third operating mode, asdirected by a computer program executed by the CPU, the audio-interfaceIC either merely relays commands and data between the computersubsystem's digital computer bus and the CD-ROM drive, or independentlyresponds to CD-ROM button commands by generating CD-ROM commandsinternally, and independently transmitting such commands to the CD-ROMdrive to control playing an audio CD present in the CD-ROM drive.

An advantage of the present invention is that during playing of audioCDs the invention can double the interval during which the digitalcomputer can operate on a particular amount of battery power.

These and other features, objects and advantages will be understood orapparent to those of ordinary skill in the art from the followingdetailed description of the preferred embodiment as illustrated in thevarious drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting a digital computer in accordancewith the present invention having both a computer subsystem and a CD-ROMsubsystem that includes an audio-interface IC;

FIG. 2 is a block diagram depicting an audio-interface IC in accordancewith the present invention;

FIG. 3 is a register diagram depicting contents of a register blockincluded in the audio-interface IC illustrated in FIG. 2;

FIG. 4 is a state diagram depicting operation of the audio-interface ICillustrated in FIG. 2 if the computer subsystem is not energized and isinoperative;

FIG. 5 is a flow diagram depicting operation of the computer togetherwith the audio-interface IC illustrated in FIG. 2 when the computersubsystem is energized and the audio-interface IC operates in the thirdoperating mode; and

FIG. 6 is a block diagram depicting a circuit preferably included in theaudio-interface IC for interfacing between a bus included in thecomputer subsystem of the digital computer and a CD-ROM drive.

DETAILED DESCRIPTION

FIG. 1, depicts a system block diagram of an exemplary battery-poweredportable computer 100 adapted for reading digital computer data from acompact disk—read only memory (“CD-ROM”), and for playing audio CDs. Forpurposes of playing audio CDs and power management, the computer 100 isfunctionally partitioned into a computer subsystem 104 and a CD-ROMsubsystem 106.

The Computer Subsystem 104

The computer subsystem 104 includes all conventional data-processingcomponents such as a microprocessor-based central processing unit andrandom access memory (“RAM”) system (“CPU system”) 120 together withvarious ICs described in greater detail below. The computer 100 alsoconventionally includes a display 110 (TFT LCD matrix display, CRT andthe like), manual input devices 112 (e.g. keyboard, mouse, touch-pad),and read-write mass storage device 114 (e.g. hard drives, floppy drives,optical drives and the like), which operate conventionally within thecomputer 100.

The computer subsystem 104 includes associated input/output (“I/O”)buses (e.g. PCI bus 116, and ISA bus 118) for interconnecting varioussubsystems included in the computer 100. In the computer 100, the CPU120 (e.g. a Pentium microprocessor) exchanges data with the PCI bus 116through a system controller IC 122 (e.g. Intel 82439HX known as the“Northbridge”) for controlling on-board L2 cache. The system controllerIC 122 is described in a document, a copy of which may be retrieved fromhttp://developer.intel.com/design/intarch/TOP_(—)800.HTM, that is herebyincorporated by reference. Via suitable adapter devices, conventionallythe PCI bus 116 permits the CPU 120 to exchange data with higherperformance devices such as the display 110 and the read-write massstorage device 114.

A bus bridge IC 124 (e.g. a 82371SB “Southbridge” IC) interconnects thePCI bus 116 with the ISA bus 118 and with an IDE bus 128. A descriptionof the bus bridge IC 124 can be obtained fromhttp://developer.intel.com/design/intarch/embdmodl.htm and is herebyincorporated by reference. The bus bridge IC 124 is available from IntelCorp., Santa Clara, Calif. Via suitable adapter devices, conventionallythe ISA bus 118 permits the CPU 120 to exchange data with lower speeddevices such as the manual input devices 112. However, for particularconfigurations of the computer 100, the CPU 120 may exchange data withhigher performance devices, such as the read-write mass storage device114, via the ISA bus 118, or directly via the IDE bus 128.

A digital-audio generation IC 130 included in the computer subsystem 104communicates with the CPU 120 either via the ISA bus 118, or via the PCIbus 116 as indicated by a pair of dashed lines 126. The digital-audiogeneration IC 130 is conventional and may be either a Maestro-1™ orMaestro-2™ marketed by ESS Technology, Inc. of Fremont, Calif.

As is well known to those skilled in the art, the computer programsexecuted by the microprocessor included in the CPU 120 of a laptop ornotebook computer 100 usually include Power Management Routines (PMRs).Under appropriate operating conditions, the PMRs may place the computer100 into one of the several power management operating modes such asthose described previously for the ACPI Specification. Computer programsthat place the computer 100 into one of the various power managementoperating modes may be prepared by one having ordinary skill in the art,and form no part of the present invention. Each of the various powermanagement operating modes is characterized by a corresponding batterypower requirement. For example, the current demand for the ACPISpecification's Device Power States may be those set forth below.

Device Power State Battery Power Required D0-Fully On Ir1 D1 Ir2 < Ir1D2 Ir3 < Ir2 D3-off Ir4 < Ir3

The CD-ROM Subsystem 106

The CD-ROM subsystem 106 includes an audio-interface IC 102 inaccordance with the present invention, a CD-ROM drive 138, CD-ROMcontrol buttons 142, an icon liquid crystal display (“LCD”) 144, atrack-number display 147, an audio output amplifier 146, and audiooutput transducer 148, e.g. speakers or headphones. The CD-ROM controlbuttons 142, which connect to audio-interface IC 102 via acontrol-button bus 143, include buttons for playing or pausing an audioCD, for fast-forwarding the audio CD, for rewinding the audio CD, andfor stopping or ejecting the audio DC. Thus, the CD-ROM control buttons142 permit a user of the computer 100 to completely control operation ofthe CD-ROM drive 138 while playing an audio CD. The CD-ROM drive 138 isa conventional CD-ROM drive capable of operating with conventional ATAPIinterface commands provided through an IDE-bus extension 129 whichoriginates at the audio-interface IC 102. The CD-ROM subsystem 106 alsoincludes an audio signals bus 152 that supplies left and right channelstereo audio signals directly to the audio output amplifier 146.

Depending upon the operating mode of the computer 100, an audio switch154, which operates in response to control signals received from theaudio-interface IC 102, may couple the left and right channel stereoaudio signals to the digital-audio generation IC 130 included in thecomputer subsystem 104. When the computer subsystem 104 is notenergized, to reduce electrical power consumption caused by leakageelectrical currents in ICs included in the computer subsystem 104:

1. the audio switch 154 electrically isolates the audio signals bus 152from the CD-ROM drive 138; and

2. the audio-interface IC 102 correspondingly electrically isolatesitself from the IDE bus 128.

A loudness control-signal line 156 couples a volume control signal fromthe audio-interface IC 102 to the audio output amplifier 146. Asdescribed in greater detail below, the audio-interface IC 102 provides asignal to the icon LCD 144 via a LCD-signal line 145 for indicating thatthe CD-ROM drive 138 is operating. The audio-interface IC 102 providessignals to the track-number display 147 via a track-number-display bus149 for displaying a track number as an audio CD is played.

A System Management Bus (“SMBus”) 162 permits the audio-interface IC 102to exchange commands and data with the computer subsystem 104. Withinthe computer subsystem 104, the SMBus 162 connects to the ISA bus 118via a keyboard controller IC 164 as indicated in FIG. 1, or the SMBus162 may connect directly to the bus bridge IC 124, as indicated by adashed line 166 in FIG. 1.

Both the computer subsystem 104 and the CD-ROM subsystem 106 receiveelectrical power directly from a battery, not illustrated in any of theFIGS. Depending upon the operating mode of the computer 100 for playingaudio CDs, either the CD-ROM subsystem 106 alone, or both the computersubsystem 104 and the CD-ROM subsystem 106 may be energized. If thecomputer subsystem 104 receives no electrical power, then operation ofthe CD-ROM drive 138 is effected completely within the CD-ROM subsystem106 with the audio-interface IC 102 originating signals for controllingoperation of the CD-ROM drive 138. If the computer subsystem 104 isenergized and operating, then operation of the CD-ROM drive 138 can beeffected, via the audio-interface IC 102, by commands received from acomputer program executed by the 120.

Interconnections Between the Computer Subsystem 104 and the CD-ROMSubsystem 106

The audio-interface IC 102 exchanges commands and data with a computerprogram executed by the CPU 120 through the IDE bus 128 and the SMBus162. The computer subsystem 104 also selectively supplies left and rightaudio signals from the CD-ROM drive 138 to the digital-audio generationIC 130 depending upon the operating mode of the computer 100. Duringoperation of the computer 100 in which the computer subsystem 104 isenergized and operating, the audio-interface IC 102 relays commands anddata between the computer subsystem 104 and CD-ROM drive 138. Commandsand data which the computer subsystem 104 exchanges with the CD-ROMdrive 138 are well known to those skilled in the art.

General Description of the Audio-Interface IC 102

Referring now to a detailed block diagram of the audio-interface IC 102depicted in FIG. 2, the audio-interface IC 102 includes a state machine202 which controls overall operation of the audio-interface IC 102.Inclusion of the state machine 202 in the audio-interface IC 102, ascontrasted with a programmable controller, provides better performance.The state machine 202 connects via an internal bus 204 to a SMBusinterface 206, to a register block 208, to a LCD control 212, and to aclock generator 214.

The SMBus interface 206, which is coupled to the SMBus 162, issues aninterrupt signal from an INTN pin 222 whenever an user of the computer100 presses any of the CD-ROM control buttons 142. The INTN pin 222 maybe advantageously connected to a SMBus alert signal-line of the SMBus162. Upon receiving a SMBus alert signal, a computer program executed bythe CPU 120 can interrogate the register block 208 included in theaudio-interface IC 102 via the SMBus 162 to determine which of theCD-ROM control buttons 142 has been pressed.

In addition to being coupled to the internal bus 204, the register block208 is also coupled to an operating-mode bus 226 by which the computer100 supplies operating mode signals to the audio-interface IC 102. Thesignals supplied to the audio-interface IC 102 via the operating-modebus 226 include a reset signal which upon activation causes theaudio-interface IC 102 to be reset. The operating-mode bus 226 alsosupplies a signal to the audio-interface IC 102 that indicates whetherthe computer subsystem 104 is energized. And, the operating-mode bus 226supplies a signal to the audio-interface IC 102 which indicates if thedevice connected to the IDE-bus extension 129 is a CD-ROM drive.Supplying a signal to the audio-interface IC 102 which indicates whethera CD-ROM drive is connected to the IDE-bus extension 129 avoids arequirement that the audio-interface IC 102 first power on and theninterrogate a device connected to the IDE-bus extension 129 to determinewhether such device is a CD-ROM drive or a hard disk drive. Avoidingthis interrogation requirement is particularly advantageous if thecomputer 100 permits a user to readily interchange a CD-ROM drive forsome other device such as a hard disk drive or conversely.

As is readily apparent, the CD-ROM control buttons 142, icon LCD 144 andtrack-number display 147 of the CD-ROM subsystem 106 provide a user ofthe computer 100 with a self-contained interface for playing audio CDs.To effect this functionality, the audio-interface IC 102 includescontrol-button logic 232 that receives electrical signals from theCD-ROM control buttons 142 via the control-button bus 143. In responseto such signals, the control-button logic 232 may store data into theregister block 208, or it may cause a digital volume control 236 totransmit control signals via the loudness control-signal line 156 to theaudio output amplifier 146. Similarly, when the computer subsystem 104is playing an audio CD the LCD control 212 transmits a signal via theLCD-signal line 145 to activate an audio playback icon included in theicon LCD 144. And during such audio CD playback the LCD control 212transmits signals via the track-number-display bus 149 which cause thetrack-number display 147 to display a number that indicates the currenttrack of an audio CD. As described in greater detail below, dataindicating the current track number is present in the register block208.

When the computer subsystem 104 is energized, a host IDE interface 242included in the audio-interface IC 102 couples electrical signalsbetween the IDE bus 128 and the IDE-bus extension 129 via an IDE-signalsmultiplexer 244 included in the audio-interface IC 102. The electricalsignals coupled between the IDE bus 128 and the IDE-bus extension 129effect exchanges of commands and data between a computer programexecuted by the CPU 120 and the CD-ROM drive 138. However, if only theCD-ROM subsystem 106 is energized, then the state machine 202 exchangeselectrical signals with the IDE-bus extension 129 via the IDE-signalsmultiplexer 244. And as set forth above, if the computer subsystem 104is not energized the host IDE interface 242 electrically isolates theaudio-interface IC 102 from the IDE bus 128 of the computer subsystem104 to reduce, as much as possible, leakage current flowing from theCD-ROM subsystem 106 into the computer subsystem 104. Set forth below isa table listing commands in accordance with the ATAPI protocol which thestate machine 202 may transmit to the CD-ROM drive 138 via theIDE-signals multiplexer 244 and IDE-bus extension 129.

Command Opcode Pause/Resume 4Bh Play Audio MSF 47h Stop 1Bh Read Tableof Contents 43h Request Sense 03h Read Sub Channel 42h Test Unit Ready00h Lock/Unlock 1Eh Inquiry 12h Sleep E6h

The state machine 202 also transmits a pair of signals from theaudio-interface IC 102 via a mode-control signal bus 246. A mode signalsupplied by the state machine 202 to the mode-control signal bus 246indicates that the audio-interface IC 102 is operating for receivingsignals from the CD-ROM control buttons 142. An electrical power controlsignal supplied by the state machine 202 to the mode-control signal bus246 indicates either that the CD-ROM drive 138 is or has been recentlyoperating, or that the CD-ROM drive 138 has not been operating recently.This signal may be used within the CD-ROM subsystem 106 forappropriately controlling the supply of electrical power to the CD-ROMdrive 138 and/or the audio output amplifier 146.

During power-on initialization of the computer 100, input signalssupplied to the audio-interface IC 102 via the mode-control signal bus246 respectively select an address for the audio-interface IC 102 on theSMBus 162, and also control whether the state machine 202 transmits thesignal for controlling the supply of electrical power to the CD-ROMdrive 138 and/or the audio output amplifier 146.

The audio-interface IC 102 also receives a pair of signals via amode-set signal bus 248 that specify particular hardware characteristicsof the CD-ROM subsystem 106. One of these signals specifies which one oftwo (2) different types of volume control ICs is included in the audiooutput amplifier 146. The other mode-set signal specifies a particularcharacteristic for signals transmitted from the audio-interface IC 102to the track-number display 147 via the track-number-display bus 149.

The clock generator 214 included in the audio-interface IC 102 connectsto an oscillator-in signal-line 252 and to an oscillator-our signal-line254. The oscillator-in and -out signal-lines 252 and 254 connect to an 8MHz crystal external to the audio-interface IC 102 that is notseparately depicted in any of the FIGS.

Register Block 208

FIG. 3 illustrates registers 208 a- 208 h included in the register block208. Seven high order bits of a low-order byte 208 aa of a Chip andRevision ID Register 208 a stores a programmable address for theaudio-interface IC 102 on the SMBus 162. A high-order byte 208 ab of theChip and Revision ID Register 208 a stores a revision number for theaudio-interface IC 102.

Respective states stored in software programmable bits 0, 1, 3, and 5 ina low-order byte of a Control-Buttons Change-Register 208 b store dataindicating pressing of specific CD-ROM control buttons 142. Bit 208 ba 0in the stores data indicating that a rewind button has been pressed. Bit208 ba 1 stores data indicating that a fast-forward button has beenpressed. Bit 208 ba 3 in the Control-Buttons Change-Register 208 bstores data indicating that a stop/eject button has been pressed. Andbit 208 ba 5 in the Control-Buttons Change-Register 208 b stores dataindicating that a play/pause button has been pressed. Softwareprogrammable bit 208 hb 0 of a high-order byte of an Interrupt StatusRegister 208 h stores data indicating that one of the CD-ROM controlbuttons 142 has been pressed, and causes the interrupt signal to betransmitted from the audio-interface IC 102 via the INTN pin 222.Because bits 0, 1, 3 and 5 in the Control-Buttons Change-Register 208 bare software programmable, a computer program executed by the CPU 120can assign data values, i.e. 0 or 1, to any of them.

A state of bit 208 ba 7 of the Control-Buttons Change-Register 208 bindicates whether the audio-interface IC 102 is enabled for transmittingthe ATAPI protocol commands listed above to the CD-ROM drive 138. Astate of bit 208 ca 1 in a low-order byte of an Electrical PowerRegister 208 c enables the audio-interface IC 102 for controllingelectrical power to the CD-ROM drive 138 and to the audio outputamplifier 146. If bit 208 ca 1 is enabled, a state of bit 208 ca 0 inthe Electrical Power Register 208 c controls the supply of electricalpower to the CD-ROM drive 138 and audio output amplifier 146.

Respective states stored in software programmable bits 0, 1 and 2 in alow-order byte of a Command Control Register 208 d store data forcontrolling operation of the audio-interface IC 102 when the computersubsystem 104 is energized. Bit 208 da 0 in the Command Control Register208 d stores data which controls whether the audio-interface IC 102 isenabled for executing a single instance of a function specified, asdescribed above, by data values that are assigned to bits in theControl-Buttons Change-Register 208 b by pressing the CD-ROM controlbuttons 142. Bit 208 da 1 stores data which prevents the audio-interfaceIC 102 from executing any function specified by data values assigned bypressing the CD-ROM control buttons 142 to bits in the Control-ButtonsChange-Register 208 b. Bit 208 da 2 stores data which indicates whetherthe audio-interface IC 102 has executed a single instance of a functionspecified by data values assigned by pressing the CD-ROM control buttons142 to bits in the Control-Buttons Change-Register 208 b.

A bit 208 ea 0 in a low-order byte of an Operating Mode Register 208 estores data which specifies a specific operating mode of the CD-ROMsubsystem 106, i.e. the third operating mode, to be described in greaterdetail below. When the audio-interface IC 102 operates in the thirdoperating mode, bit 208 ea 4 in the Operating Mode Register 208 e storesdata which specifies selection of a particular drive, i.e. a masterdrive or a slave drive, to receive ATAPI protocol commands from theaudio-interface IC 102.

Bit 208 eb 1 in a high-order byte of the Operating Mode Register 208 econtrols application of a clock signal to the state machine 202. Whenthe audio-interface IC 102 operates in the mode in which it merelyrelays data and ATAPI commands between the IDE bus 128 and the IDE-busextension 129, to conserve energy bit 208 eb 1 may be set therebyhalting application of the clock signal to the state machine 202.

Bits 0-6 of a low-order byte 208 ga of a Track Number Register 208 gstore a track number read from an audio CD while it is being played.Software programmable bits 0-6 of a high-order byte 208 gb store a tracknumber displayed on the track-number display 147.

Operating Modes

If the computer subsystem 104 is energized and operating, in oneoperating mode of the CD-ROM subsystem 106 the audio-interface IC 102transparently relays commands and data between the IDE bus 128 and theCD-ROM drive 138. FIG. 4 depicts states and state transitions of theaudio-interface IC 102 for an operating mode of the computer 100 inwhich the computer subsystem 104 is not energized and is inoperative.When the computer subsystem 104 is not energized and the CD-ROMsubsystem 106 is initially energized, or immediately after theaudio-interface IC 102 is reset by a signal received from theoperating-mode bus 226, the audio-interface IC 102 enters an initializestate 302 depicted in FIG. 4.

A pressing of the play/pause button when the audio-interface IC 102 isin the initialize state 302 causes the audio-interface IC 102 totransition to a play state 304 in which the audio-interface IC 102transmits commands in accordance with the ATAPI protocol to the CD-ROMdrive 138 that cause the CD-ROM subsystem 106 to play an audio CD. Ifthe audio-interface IC 102 is in the play state 304, then pressing thestop or eject button, or reaching the end of all the audio CD trackscauses the audio-interface IC 102 to re-enter the initialize state 302and to return to the beginning of the audio CD.

While the CD-ROM subsystem 106 is in the play state 304 playing an audioCD, pressing either the fast-forward or rewind buttons causes theaudio-interface IC 102 to enter a fast-forward-or-rewind state 306. Inthe fast-forward-or-rewind state 306 the audio-interface IC 102transmits commands to the CD-ROM drive 138 that either fast-forward orrewind the audio CD. If the CD-ROM drive 138 completes the fast-forwardor rewind command, or reaches the end or beginning of the audio CDtrack, the audio-interface IC 102 re-enters the initialize state 302.While the CD-ROM subsystem 106 is fast-forwarding or rewinding an audioCD, pressing the play button causes the audio-interface IC 102 to enterthe play state 304 and resume playing the audio CD at the beginning ofthe present track.

While the CD-ROM subsystem 106 is in the play state 304 playing an audioCD or in the fast-forward-or-rewind state 306 fast-forwarding orrewinding an audio CD, pressing the pause button causes theaudio-interface IC 102 to enter a pause state 308 which pauses operationof the CD-ROM drive 138. If the audio-interface IC 102 is in the pausestate 308, pressing the play button causes the audio-interface IC 102 toenter the play state 304 and resume playing the audio CD at the presentlocation in the track, pressing either the fast-forward or rewindbuttons causes the audio-interface IC 102 to enter the pause state 308,and pressing the stop button causes the audio-interface IC 102 to enterthe initialize state 302.

If the audio-interface IC 102 is in the initialize state 302, and asignal has been supplied to the audio-interface IC 102 via the mode-setsignal bus 248 which enables the state machine 202 for controlling thesupply of electrical power to the CD-ROM drive 138 and/or the audiooutput amplifier 146, and a pre-established two (2) minute intervalpasses during which none of the CD-ROM control buttons 142 are pressed;then the audio-interface IC 102 enters a sleep state 312. Upon enteringthe sleep state 312 the audio-interface IC 102 sends an ATAPI protocolsleep command to the CD-ROM drive 138 thereby slowing down a clockincluded in the CD-ROM drive 138. If the audio-interface IC 102 is inthe sleep state 312 and a second, pre-established two (2) minuteinterval passes during which none of the CD-ROM control buttons 142 arepressed, then the audio-interface IC 102 enters a suspend state 314 inwhich the audio-interface IC 102 transmits a signal via the mode-controlsignal bus 246 which indicates that the CD-ROM drive 138 has not beenoperating recently. Electrical circuitry included in the CD-ROMsubsystem 106 may use this signal from the audio-interface IC 102 forremoving electrical power from both the CD-ROM drive 138 and from theaudio output amplifier 146. If the audio-interface IC 102 is either inthe sleep state 312 or in the suspend state 314, then pressing any ofthe CD-ROM control buttons 142 causes the audio-interface IC 102 tore-enter the initialize state 302.

In addition to the two operating modes described above, the preferredembodiment of the audio-interface IC 102 may be configured to operate athird operating mode. In this third operating mode the computersubsystem 104 is energized and operating, the audio-interface IC 102receives commands from the CD-ROM control buttons 142, and stores suchcommands into the Control-Buttons Change-Register 208 b for subsequentretrieval by a computer program executed by the CPU 120. The flowdiagram of FIG. 5 depicts operation of the computer 100 including theaudio-interface IC 102 for playing audio CDs in this third operatingmode. Thus as depicted in FIG. 5, in the third operating mode while auser does not press any of the CD-ROM control buttons 142 the statemachine 202 loops at decision block 372 waiting for one of the CD-ROMcontrol buttons 142 to be pressed. If any of the CD-ROM control buttons142 are pressed, the state machine 202 in processing block 374 sets thebit 208 hb 0 in the Interrupt Status Register 208 h thereby causing theSMBus interface 206 to transmit a SMBus interrupt INTN via the INTN pin222 either to the keyboard controller IC 164, or to the bus bridge IC124. In addition to transmitting a SMBus interrupt, the state machine202 in processing block 374 also sets the appropriate bit in theControl-Buttons Change-Register 208 b to indicate which of the CD-ROMcontrol buttons 142 has been pressed. The computer program executed bythe CPU 120 in processing block 376 responds to receipt of the INTNinterrupt by reading the contents of the Control-Buttons Change-Register208 b, and by then transmitting data to the CPU 120 via the SMBus 162that resets the bit 208 hb 0 of the Interrupt Status Register 208 hthereby clearing the INTN interrupt.

After the computer program executed by the CPU 120 clears the INTNinterrupt, the state machine 202 in decision block 378 determineswhether the computer program executed by the CPU 120 has previously setbit 208 da 1 in the Command Control Register 208 d. If bit 208 da 1 isnot set, then the state machine 202 in decision block 382 determineswhether the computer program executed by the CPU 120 has previously setbit 208 da 0 in the Command Control Register 208 d. If bit 208 da 0 isset, then the state machine 202 in processing block 384 transmits to theCD-ROM drive 138 via the IDE-bus extension 129 the ATAPI command(s)which cause the IDE bus 128 to respond appropriately to the pressing ofthe CD-ROM control button 142. If bit 208 da 1 has been set, and bit 208da 0 has not been set, then the state machine 202 merely passes throughprocessing block 386 without sending any commands to the CD-ROM drive138.

After performing either processing block 384 or processing block 386,the state machine 202 in processing block 392 again sets bit 208 hb 0 inthe Interrupt Status Register 208 h thereby again causing the SMBusinterface 206 to transmit a SMBus interrupt via the INTN pin 222 eitherto the keyboard controller IC 164, or to the bus bridge IC 124. Asbefore, the computer program executed by the CPU 120 in processing block394 responds to receipt of the INTN interrupt by transmitting data tothe CPU 120 via the SMBus 162 that resets bit 208 hb 0 of the InterruptStatus Register 208 h thereby clearing the INTN interrupt. After thecomputer program executed by the CPU 120 resets bit 208 hb 0, the statemachine 202 returns to decision block 372 to resume waiting for one ofthe CD-ROM control buttons 142 to be pressed.

FIG. 6 depicts a preferred circuit for interfacing between the IDE bus128, the IDE-bus extension 129 and the audio-interface IC 102. While thehost IDE interface 242 and the IDE-signals multiplexer 244 of theaudio-interface IC 102 may employ conventional IC I/O buffers, suchconventional circuits will appreciably delay signals passing through theaudio-interface IC 102 between the IDE bus 128 and the IDE-bus extension129. Rather than employing conventional IC I/O buffer circuits for thehost IDE interface 242 and the IDE-signals multiplexer 244, thepreferred circuit depicted in FIG. 6 employs transmission gates 402interposed between the IDE bus 128 and the IDE-bus extension 129 forselectively either coupling them together, or isolating them from eachother. Use of the transmission gates 402 rather than conventional IC I/Obuffers significantly reduces delay for signals passing through theaudio-interface IC 102 between the IDE bus 128 and the IDE-bus extension129.

Analogously to the transmission gates 402, signals that the statemachine 202 receives from the IDE-bus extension 129 are also selectivelycoupled to or isolated from input buffers 404 for the state machine 202by a transmission gates 406 also in response to a signal generated bythe state machine 202. Alternatively, signals that the state machine 202transmits to the IDE-bus extension 129 pass through output drivers 408when the output drivers 408 are enabled by a control signal supplied bythe state machine 202. This preferred configuration for exchangingsignals between the state machine 202 and the IDE-bus extension 129 alsopermits isolating the state machine 202 from input signals on theIDE-bus extension 129 during certain critical events such as when theCD-ROM drive 138 is removed to be replaced by another device that is tobe coupled to the IDE bus 128 without removing electrical power from thecomputer subsystem 104 and CD-ROM subsystem 106.

The transmission gates 402, input buffers 404, transmission gates 406,and output drivers 408 collectively constitute an analog multiplexerthat, in the illustration of FIG. 6, is enclosed within a dashed line412. Enabling operation of the analog multiplexer 412 permits exchangingsignals between the IDE-bus extension 129 and either the IDE bus 128 orstate machine 202. Disabling operation of the analog multiplexer 412isolates the IDE-bus extension 129 both from the IDE bus 128 and fromthe state machine 202. Inclusion of the analog multiplexer 412 in theaudio-interface IC 102 permits various IDE devices, such as the CD-ROMdrive 138, a hard disk, a Digital Video Disk (“DVD”) drive, a ZIP drive,or a Superdisk, to be connected to or disconnected from the IDE-busextension 129 of a fully operational computer 100.

Operation of the analog multiplexer 412 is enabled or disabled by asignal coupled from a multiplexer control 414 included in theaudio-interface IC 102. The multiplexer control 414 transmits thecontrol signal to the transmission gates 402 and 406 via atransmission-gate-control signal-line 416. For disabling operation ofthe analog multiplexer 412, the multiplexer control 414 receivessoftware data via the SMBus 162. For enabling operation of the analogmultiplexer 412, the multiplexer control 414 may receive either or botha hardware signal via a touchdown signal-line 418 and/or software datavia the SMBus 162. The multiplexer control 414 also receives signalsfrom the state machine 202 via a multiplexer-control-signalselection-bus 422. Signals supplied to the multiplexer control 414 fromthe state machine 202 via the multiplexer-control-signal selection-bus422 determine whether operation of the multiplexer control 414 isenabled by the signal supplied via the touchdown signal-line 418, or bydata supplied to the multiplexer control 414 via the SMBus 162.

Accordingly, as illustrated in FIG. 3 the register block 208 of theaudio-interface IC 102 which employs the preferred analog multiplexer412 includes a software programmable bit 208 ca 3 located in thelow-order byte of the Electrical Power Register 208 c. Setting bit 208ca 3 activates software data control of the multiplexer control 414 viathe SMBus 162, while resetting bit 208 ca 3 activates hardwareenablement of the multiplexer control 414 via the touchdown signal-line418. To effect software disabling and enabling of the multiplexercontrol 414, the low-order byte of the Electrical Power Register 208 calso includes a software programmable bit 208 ca 2 which if set enablesoperation of the transmission gates 402 and 406. Conversely, resettingbit 208 ca 2 disables operation of the transmission gates 402 and 406.

When the computer 100 is fully operational with a device connected tothe IDE-bus extension 129, using one of the manual input devices 112 auser of the computer 100 may direct a computer program executed by theCPU 120 to release the device connected to the IDE-bus extension 129. Inresponse thereto, the computer 100 causes the analog multiplexer 412 toisolate the IDE-bus extension 129 from the remainder of the CD-ROMsubsystem 106, and then mechanically releases the device so it may bephysically removed from the computer 100. Upon subsequent insertion of adevice into the computer 100 and connection of that device to theIDE-bus extension 129, the device becomes mechanically locked into thecomputer 100, and the audio-interface IC 102 responsive, either thehardware signal or software data, re-couples the IDE-bus extension 129to the remainder of the CD-ROM subsystem 106 thereby restoring thecomputer 100 to full operation.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is purely illustrative and is not to be interpreted aslimiting. Consequently, without departing from the spirit and scope ofthe invention, various alterations, modifications, and/or alternativeapplications of the invention will, no doubt, be suggested to thoseskilled in the art after having read the preceding disclosure.Accordingly, it is intended that the following claims be interpreted asencompassing all alterations, modifications, or alternative applicationsas fall within the true spirit and scope of the invention.

What is claimed is:
 1. A digital computer comprising both a computersubsystem and a CD-ROM subsystem: said computer subsystem including adigital computer bus via which commands and data are exchanged betweenvarious pairs of digital computer devices of the computer subsystem thatare selected from a group consisting of a central processing unit(“CPU”), a random access memory (“RAM”), a display, a read-write massstorage device, a manual input device, and a digital-audio generatingintegrated circuit (“IC”); and said CD-ROM subsystem including: a CD-ROMdrive; an audio output amplifier that is coupled to the CD-ROM drive forreceiving an analog audio signal from the CD-ROM drive; a plurality ofCD-ROM control buttons for controlling operation of the CD-ROM driveduring replay of audio compact disks (“CDs”); and an audio-interface ICthat is continuously connected to the digital computer bus of saidcomputer subsystem, to a bus extension included in said CD-ROM subsystemto which the CD-ROM drive connects, and to the CD-ROM control buttons,said audio-interface IC comprising transmission gates to selectivelycouple or isolate said digital computer bus and said bus extension; theaudio-interface IC when the CD-ROM subsystem is energized: in a firstoperating mode in which the computer subsystem is energized andoperating, relaying commands and data between the digital computer busof the computer subsystem and the CD-ROM drive; and in a secondoperating mode in which the computer subsystem is not energized and isinoperative, autonomously responding to signals received from the CD-ROMcontrol buttons and transmitting commands to the CD-ROM drive, thecommands causing the CD-ROM drive to play an audio CD present in theCD-ROM drive.
 2. The digital computer of claim 1 wherein said CD-ROMsubsystem further comprises an audio switch, which operates in responseto a control signal received from the audio-interface IC, for couplingthe analog audio signal of the CD-ROM drive to the digital-audiogenerating IC included in the computer subsystem when theaudio-interface IC operates in the first operating mode, and forisolating the analog audio signal of the CD-ROM drive from thedigital-audio generating IC included in the computer subsystem when theaudio-interface IC operates in the second operating mode.
 3. The digitalcomputer of claim 1 wherein said CD-ROM subsystem further comprises anicon display that operates in response to a control signal received fromthe audio-interface IC for indicating operating status of the CD-ROMdrive.
 4. The digital computer of claim 1 wherein said CD-ROM subsystemfurther comprises a track-number display that operates in response tocontrol signals received from the audio-interface IC for indicatingplaying status of an audio CD present in the CD-ROM drive.
 5. Thedigital computer of claim 1 wherein during operation of theaudio-interface IC in the second operating mode, when the CD-ROM drivehas been idle for a first pre-established interval of time, theaudio-interface IC enters a lower power operating mode, and transmits acommand to the CD-ROM drive that places the CD-ROM drive in a lowerpower operating mode.
 6. The digital computer of claim 5 wherein duringoperation of the audio-interface IC in the second operating mode, whenthe CD-ROM drive has been idle for a second pre-established interval oftime, the audio-interface IC turns off electrical power to the CD-ROMdrive.
 7. The digital computer of claim 6 wherein the audio-interface ICturns off electrical power to the audio output amplifier upon turningoff electrical power to the CD-ROM drive.
 8. The digital computer ofclaim 1 wherein the audio-interface IC supplies a volume control signalto the audio output amplifier.
 9. The digital computer of claim 1wherein the computer subsystem further comprises a System Management Bus(“SMBus”), the audio-interface IC exchanging commands and data with thecomputer subsystem via the SMBus.
 10. The digital computer of claim 1wherein the audio-interface IC has a third operating mode in which thecomputer subsystem is energized and operating and the audio-interfaceIC, as permitted by a computer program executed by the CPU included insaid computer subsystem, responds to signals received from the CD-ROMcontrol buttons and transmits commands to the CD-ROM drive, the commandscausing the CD-ROM drive to play an audio CD present in the CD-ROMdrive.
 11. The digital computer of claim 1 wherein said audio-interfaceIC includes a multiplexer that when enabled couples the CD-ROM drive tothe digital computer bus of the computer subsystem, and that whendisabled isolates the CD-ROM drive from the digital computer bus of thecomputer subsystem thereby permitting removal and replacement of theCD-ROM drive while the computer subsystem is energized and operating.12. An audio-interface IC adapted for use in a digital computer havingboth a computer subsystem and a CD-ROM subsystem: the computer subsystemincluding a digital computer bus via which commands and data areexchanged between various pairs of digital computer devices of thecomputer subsystem that are selected from a group consisting of a CPU, aRAM, a display, a read-write mass storage device, and a manual inputdevice; and the CD-ROM subsystem including: a CD-ROM drive; an audiooutput amplifier that is coupled to the CD-ROM drive for receiving ananalog audio signal from the CD-ROM drive; and a plurality of CD-ROMcontrol buttons for controlling operation of the CD-ROM drive duringreplay of audio disks; the audio-interface IC comprising: a host-businterface that is adapted to continuously connect the audio-interface ICwith the digital computer bus of the computer subsystem, said host-businterface comprising transmission gates to selectively couple or isolatesaid digital computer bus and said CD-ROM drive; a CD-ROM interface thatis adapted to continuously connect the audio-interface IC with a busextension included in the CD-ROM to which the CD-ROM drive connects; acontrol-button logic adapted for coupling the audio-interface IC to theCD-ROM control buttons and for receiving electrical signals from theCD-ROM control buttons; the audio-interface IC when the CD-ROM subsystemis energized: in a first operating mode in which the computer subsystemis energized and operating, relaying commands and data between thedigital computer bus of the computer subsystem and the CD-ROM drive; andin a second operating mode in which the computer subsystem is notenergized and is inoperative, autonomously responding to signalsreceived from the CD-ROM control buttons and transmitting commands tothe CD-ROM drive, the commands causing the CD-ROM drive to play an audioCD present in the CD-ROM drive.
 13. The audio-interface IC of claim 12wherein the CD-ROM subsystem further includes an audio switch, theaudio-interface IC being adapted for supplying a control signal to theaudio switch for coupling the analog audio signal of the CD-ROM drive toa digital-audio generating IC included in the computer subsystem whenthe audio-interface IC operates in the first operating mode, and forisolating the analog audio signal of the CD-ROM drive from thedigital-audio generating IC included in the computer subsystem when theaudio-interface IC operates in the second operating mode.
 14. Theaudio-interface IC of claim 12 wherein the CD-ROM subsystem furtherincludes an icon display, the audio-interface IC being adapted forsupplying a control signal to the icon display which effects anindication of an operating status of the CD-ROM drive.
 15. Theaudio-interface IC of claim 12 wherein the CD-ROM subsystem furtherincludes a track-number display, the audio-interface IC being adaptedfor supplying control signals to the track-number display for indicatingplaying status of an audio CD present in the CD-ROM drive.
 16. Theaudio-interface IC of claim 12 wherein during operation of theaudio-interface IC in the second operating mode, when the CD-ROM drivehas been idle for a first pre-established interval of time, theaudio-interface IC enters a lower power operating mode, and transmits acommand to the CD-ROM drive that places the CD-ROM drive in a lowerpower operating mode.
 17. The audio-interface IC of claim 16 whereinduring operation of the audio-interface IC in the second operating mode,when the CD-ROM drive has been idle for a second pre-establishedinterval of time, the audio-interface IC turns off electrical power tothe CD-ROM drive.
 18. The audio-interface IC of claim 17 wherein theaudio-interface IC turns off electrical power to the audio outputamplifier upon turning off electrical power to the CD-ROM drive.
 19. Theaudio-interface IC of claim 12 wherein the audio-interface IC supplies avolume control signal to the audio output amplifier.
 20. Theaudio-interface IC of claim 12 wherein the computer subsystem furtherincludes a System Management Bus (“SMBus”), the audio-interface IC beingadapted for exchanging commands and data with the computer subsystem viathe SMBus.
 21. The audio-interface IC of claim 12 wherein theaudio-interface IC has a third operating mode in which the computersubsystem is energized and operating and the audio-interface IC, aspermitted by a computer program executed by the CPU included in thecomputer subsystem, responds to signals received from the CD-ROM controlbuttons and transmits commands to the CD-ROM drive, the commands causingthe CD-ROM drive to play an audio CD present in the CD-ROM drive. 22.The audio-interface IC of claim 12 wherein the audio-interface ICincludes a multiplexer that when enabled couples the CD-ROM drive to thedigital computer bus of the computer subsystem, and that when disabledisolates the CD-ROM drive from the digital computer bus of the computersubsystem thereby permitting removal and replacement of the CD-ROM drivewhile the computer subsystem is energized and operating.
 23. A digitalcomputer comprising both a computer subsystem and a CD-ROM subsystem:said computer subsystem including a digital computer bus via whichcommands and data are exchanged between various pairs of digitalcomputer devices of the computer subsystem that are selected from agroup consisting of a CPU, a RAM, a display, a read-write mass storagedevice, a manual input device, and a digital-audio generating IC; andsaid CD-ROM subsystem including: a CD-ROM drive; an audio outputamplifier that is coupled to the CD-ROM drive for receiving an analogaudio signal from the CD-ROM drive; a plurality of CD-ROM controlbuttons for controlling operation of the CD-ROM drive during replay ofaudio CDs; and an audio-interface IC that is coupled to the digitalcomputer bus of said computer subsystem, to the CD-ROM drive, and to theCD-ROM control buttons, said audio-interface IC comprising transmissiongates to selectively couple or isolate said digital computer bus andsaid CD-ROM drive; the audio-interface IC when the CD-ROM subsystem isenergized: in a first operating mode in which the computer subsystem isenergized and operating, relaying commands and data between the digitalcomputer bus of the computer subsystem and the CD-ROM drive; and in asecond operating mode in which the computer subsystem is not energizedand is inoperative, autonomously responding to signals received from theCD-ROM control buttons and transmitting commands to the CD-ROM drive,the commands causing the CD-ROM drive to play an audio CD present in theCD-ROM drive, and when the CD-ROM drive has been idle for a firstpre-established interval of time, the audio-interface IC entering alower power operating mode, and transmitting a command to the CD-ROMdrive that places the CD-ROM drive in a lower power operating mode. 24.The digital computer of claim 23 wherein said CD-ROM subsystem furthercomprises an audio switch, which operates in response to a controlsignal received from the audio-interface IC, for coupling the analogaudio signal of the CD-ROM drive to the digital-audio generating ICincluded in the computer subsystem when the audio-interface IC operatesin the first operating mode, and for isolating the analog audio signalof the CD-ROM drive from the digital-audio generating IC included in thecomputer subsystem when the audio-interface IC operates in the secondoperating mode.
 25. The digital computer of claim 23 wherein said CD-ROMsubsystem further comprises an icon display that operates in response toa control signal received from the audio-interface IC for indicatingoperating status of the CD-ROM drive.
 26. The digital computer of claim23 wherein said CD-ROM subsystem further comprises a track-numberdisplay that operates in response to control signals received from theaudio-interface IC for indicating playing status of an audio CD presentin the CD-ROM drive.
 27. The digital computer of claim 23 wherein duringoperation of the audio-interface IC in the second operating mode, whenthe CD-ROM drive has been idle for a second pre-established interval oftime, the audio-interface IC turns off electrical power to the CD-ROMdrive.
 28. The digital computer of claim 27 wherein the audio-interfaceIC turns off electrical power to the audio output amplifier upon turningoff electrical power to the CD-ROM drive.
 29. The digital computer ofclaim 23 wherein the audio-interface IC supplies a volume control signalto the audio output amplifier.
 30. The digital computer of claim 23wherein the computer subsystem further comprises a SMBus, theaudio-interface IC exchanging commands and data with the computersubsystem via the SMBus.
 31. The digital computer of claim 23 whereinthe audio-interface IC has a third operating mode in which the computersubsystem is energized and operating and the audio-interface IC, aspermitted by a computer program executed by the CPU included in saidcomputer subsystem, responds to signals received from the CD-ROM controlbuttons and transmits commands to the CD-ROM drive, the commands causingthe CD-ROM drive to play an audio CD present in the CD-ROM drive. 32.The digital computer of claim 23 wherein said audio-interface ICincludes a multiplexer that when enabled couples the CD-ROM drive to thedigital computer bus of the computer subsystem, and that when disabledisolates the CD-ROM drive from the digital computer bus of the computersubsystem thereby permitting removal and replacement of the CD-ROM drivewhile the computer subsystem is energized and operating.
 33. Anaudio-interface IC adapted for use in a digital computer having both acomputer subsystem and a CD-ROM subsystem: the computer subsystemincluding a digital computer bus via which commands and data areexchanged between various pairs of digital computer devices of thecomputer subsystem that are selected from a group consisting of a CPU, aRAM, a display, a read-write mass storage device, and a manual inputdevice; and the CD-ROM subsystem including: a CD-ROM drive; an audiooutput amplifier that is coupled to the CD-ROM drive for receiving ananalog audio signal from the CD-ROM drive; and a plurality of CD-ROMcontrol buttons for controlling operation of the CD-ROM drive duringreplay of CDs; the audio-interface IC comprising: a host-bus interfaceadapted for coupling the audio-interface IC to the digital computer busof the computer subsystem, said host-bus interface IC comprisingtransmission gates to selectively couple or isolate said digitalcomputer bus and said CD-ROM drive; a CD-ROM interface adapted forcoupling the audio-interface IC to the CD-ROM drive; a control-buttonlogic adapted for coupling the audio-interface IC to the CD-ROM controlbuttons and for receiving electrical signals from the CD-ROM controlbuttons; the audio-interface IC when the CD-ROM subsystem is energized:in a first operating mode in which the computer subsystem is energizedand operating, relaying commands and data between the digital computerbus of the computer subsystem and the CD-ROM drive; and in a secondoperating mode in which the computer subsystem is not energized and isinoperative, autonomously responding to signals received from the CD-ROMcontrol buttons and transmitting commands to the CD-ROM drive, thecommands causing the CD-ROM drive to play an audio CD present in theCD-ROM drive, and when the CD-ROM drive has been idle for a firstpre-established interval of time, the audio-interface IC entering alower power operating mode, and transmitting a command to the CD-ROMdrive that places the CD-ROM drive in a lower power operating mode. 34.The audio-interface IC of claim 33 wherein the CD-ROM subsystem furtherincludes an audio switch, the audio-interface IC being adapted forsupplying a control signal to the audio switch for coupling the analogaudio signal of the CD-ROM drive to a digital-audio generating ICincluded in the computer subsystem when the audio-interface IC operatesin the first operating mode, and for isolating the analog audio signalof the CD-ROM drive from the digital-audio generating IC included in thecomputer subsystem when the audio-interface IC operates in the secondoperating mode.
 35. The audio-interface IC of claim 33 wherein theCD-ROM subsystem further includes an icon display, the audio-interfaceIC being adapted for supplying a control signal to the icon displaywhich effects an indication of an operating status of the CD-ROM drive.36. The audio-interface IC of claim 33 wherein the CD-ROM subsystemfurther includes a track-number display, the audio-interface IC beingadapted for supplying control signals to the track-number display forindicating playing status of an audio CD present in the CD-ROM drive.37. The audio-interface IC of claim 33 wherein during operation of theaudio-interface IC in the second operating mode, when the CD-ROM drivehas been idle for a second pre-established interval of time, theaudio-interface IC turns off electrical power to the CD-ROM drive. 38.The audio-interface IC of claim 37 wherein the audio-interface IC turnsoff electrical power to the audio output amplifier upon turning offelectrical power to the CD-ROM drive.
 39. The audio-interface IC ofclaim 33 wherein the audio-interface IC supplies a volume control signalto the audio output amplifier.
 40. The audio-interface IC of claim 33wherein the computer subsystem further includes a SMBus, theaudio-interface IC being adapted for exchanging commands and data withthe computer subsystem via the SMBus.
 41. The audio-interface IC ofclaim 33 wherein the audio-interface IC has a third operating mode inwhich the computer subsystem is energized and operating and theaudio-interface IC, as permitted by a computer program executed by theCPU included in the computer subsystem, responds to signals receivedfrom the CD-ROM control buttons and transmits commands to the CD-ROMdrive, the commands causing the CD-ROM drive to play an audio CD presentin the CD-ROM drive.
 42. The audio-interface IC of claim 33 wherein theaudio-interface IC includes a multiplexer that when enabled couples theCD-ROM drive to the digital computer bus of the computer subsystem, andthat when disabled isolates the CD-ROM drive from the digital computerbus of the computer subsystem thereby permitting removal and replacementof the CD-ROM drive while the computer subsystem is energized andoperating.